The present invention relates to controlling inductive power transfer systems for use, for example, to power portable electrical or electronic devices.
This application claims priority from the applicant's copending applications GB 0410503.7 filed on 11 May 2004 and GB 0502775.0 filed on 10 Feb. 2005, the entire content of each of which is incorporated herein by reference.
Inductive power transfer systems suitable for powering portable devices may consist of two parts:                A primary unit having at least one primary coil, through which it drives an alternating current, creating a time-varying magnetic flux.        A secondary device, separable from the primary unit, containing a secondary coil. When the secondary coil is placed in proximity to the time-varying flux created by the primary coil, the varying flux induces an alternating current in the secondary coil, and thus power may be transferred inductively from the primary unit to the secondary device.        
Generally, the secondary device supplies the transferred power to an external load, and the secondary device may be carried in or by a host object which includes the load. For example the host object may be a portable electrical or electronic device having a rechargeable battery or cell. In this case the load may be a battery charger circuit for charging the battery or cell. Alternatively, the secondary device may be incorporated in such a rechargeable cell or battery, together with a suitable battery charger circuit.
A class of such an inductive power transfer systems is described in our United Kingdom patent publication GB-A-2388716. A notable characteristic of this class of systems is the physically “open” nature of the magnetic system of the primary unit—a significant part of the magnetic path is through air. This is necessary in order that the primary unit can supply power to different shapes and sizes of secondary device, and to multiple secondary devices simultaneously. Another example of such an “open” system is described in GB-A-2389720.
Such systems may suffer from some problems. A first problem is that the primary unit cannot be 100% efficient. For example, switching losses in the electronics and I2R losses in the primary coil dissipate power even when there is no secondary device present, or when no secondary devices that are present require charge. This wastes energy. Preferably the primary unit should enter a low-power “standby mode” in this situation.
A second problem in such systems is that it is not possible to mechanically prevent foreign objects from being placed into proximity with the primary coil, coupling to the coil. Foreign objects made of metal will have eddy-currents induced therein. These eddy currents tend to act to exclude the flux, but because the material has resistance, the flowing eddy currents will suffer I2R losses which will cause heating of the object. There are two particular cases where this heating may be significant:                If the resistance of any metal is high, for example if it is impure or thin.        If the material is ferromagnetic, for example steel. Such materials have high permeability, encouraging a high flux density within the material, causing large eddy currents and therefore large I2R losses.        
In the present application, such foreign objects that cause power drain are termed “parasitic loads”. Preferably the primary unit should enter a “shutdown mode” when parasitic loads are present, to avoid heating them.
Various approaches to solve these two problems have been proposed in the prior art.
Solutions to the first problem, of not wasting power when no secondary device requires charge, include:                In EP0533247 and U.S. Pat. No. 6,118,249 the secondary device modulates its inductive load during charging, causing a corresponding variation in the power taken from the primary unit. This indicates to the primary unit that it should stay out of the standby state.        In EP1022840 the primary unit varies the frequency of its drive, and thus the coupling factor to a tuned secondary unit. If the secondary unit is not taking power, there is no difference in the power taken as the frequency is swept and thus the primary unit goes into a standby state.        In U.S. Pat. No. 5,536,979 the primary unit simply measures the power flowing in the primary coil, and enters a pulsing standby state if this is below a threshold.        In U.S. Pat. No. 5,896,278 the primary unit contains detecting coils which have power coupled back into them according to the position of the secondary device. If the device is not present the primary unit enters a standby mode.        In U.S. Pat. No. 5,952,814 the secondary device has a mechanical protrusion which fits a slot in the primary unit, activating it.        In U.S. Pat. No. 6,028,413 the primary unit drives two coils, and there are a corresponding two power receiving secondary coils in the secondary unit. The primary unit measures the power delivered from each primary coil and enters standby mode if it is below a threshold.        
Solutions to the second problem, of parasitic loads, include:                As mentioned above, in EP1022840 the primary unit varies the frequency of its drive. In this system, the secondary device is tuned, so this frequency variation will result in a variation of the power taken from the primary unit. If the load is instead a piece of metal, then varying the frequency will not have as much effect and the primary unit will enter a shutdown state.        As mentioned above, in U.S. Pat. No. 5,952,814 a key in the secondary device activates the primary unit. The assumption is that if a secondary device is present then this will physically exclude any foreign objects.        As mentioned above, in U.S. Pat. No. 6,028,413 the primary unit supplies power to the secondary device by driving two primary coils. If the amount of power supplied by the two coils is different, the primary unit assumes that the load is not a valid secondary device and enters shutdown mode.        
These approaches all assume a 1:1 relationship between the primary unit and the secondary device. Therefore they are not sufficient for systems such as those described in GB-A-2388716 where more than one secondary device at a time may be present. For example, they would not work when there are two secondary devices present, one requiring charge and the other not.
Some of these approaches also assume that the physical or electrical presence of a valid secondary device implies that all foreign objects are physically excluded by the secondary device. This is not necessarily the case, particularly when the secondary devices may be positioned arbitrarily in respect of the primary unit, as in those described in GB-A-2388716.